In gyro-optical objective systems and other optical telescope applications it is frequently desirable to scan the field of a radiation sensor across a field of an optical objective to determine the orientation of the radiation source relative to the axis of the optical system. Of the various scanning patterns available, the rosette scan is particularly advantageous in that a radiation sensor with a relatively small field of view can be scanned across the entire field of the optical objective to produce a circular search pattern with a relatively large field of view. Of the variety of prior art systems utilized to produce a rosette scanning pattern, it has been found that the use of two rotating mirrors eliminates or minimizes the difficulty encountered in the use of prismatic elements. Chromatic and other aberrations are often encountered with refractive prismatic elements and are largely uncorrectable due to the rotational nature of the principle axis of these aberrations. Also, the rotating mirrors are not limited in their effective wavelength regions as are prismatic elements.
In the prior art, the packaging of a counter rotating mirror system has presented certain problems that until the development of the present invention have lacked a solution. In most prior art systems where the primary and secondary mirrors rotate countercurrently, separate drive systems are necessary. Commonly, smaller secondary mirrors have been driven by an electric motor that is independent of the primary mirror drive system. The prior art use of electric motors has required sliding contacts such as motor brushes, grounding brushes for electro-static noise reduction or a slip ring assemblies. These components are highly undesirable in the environment of a radiation seeker head. The varying resistance at the sliding contacts induces noise. Also, the wear and associated limited life and unreliability of the typical miniature high speed brush assemblies may degrade the performance of the seeker head.
A particular prior art scanning system utilizes a non-rotating electro-magnet. The electro-magnet is used to oscillate a scanning mirror rather than rotating the mirror. A separate drive motor is mounted in the extreme aft part of the apparatus to rotate the mirror. This system is ineffective for use in a seeker head because the most efficient use of a seeker head calls for fully gimbaling all of the optical elements. The drive motor in this prior art structure is not capable of a gimbal mounting and unnecessarily complicates the structure of the seeker head.
In yet another prior art device, an optical scanner is mounted within a flywheel, the flywheel being utilized to stabilize the vehicle. In this device the stator and the core are non-rotating and mounted behind a spinning optical member. This device is not designed to seek a moving optical objective and accordingly, is not gimbled.
It is therefore desirable to have an optical scanning system in which the electric motor that drives the scanning mirror is directly coupled to the drive power supply thus eliminating the need for slip rings brushes and the like. It is likewise desirable that the system be fully gimbaled to continually track an optical objective that is changing its position relative to the seeker head.